Method of preparing a stabilized nitrocellulose article

ABSTRACT

A method for preparing a stabilized nitrocellulose article includes providing a nitrocellulose-containing mat of fibers. The nitrocellulose-containing mat of fibers is treated with a solution that comprises a solvent and a stabilizer to produce the article. The stabilizer is soluble in the solvent. The solvent dissolves the stabilizer and the nitrocellulose in the nitrocellulose-containing mat of fibers is substantially insoluble in the solvent at ambient pressure and temperature. An inflatable bag comprises a shell and the stabilized nitrocellulose article. The shell is formed from a foldable material and has an outer surface and an inner surface. The inner surface defines an interior cavity of the inflatable bag. The stabilized nitrocellulose article is disposed upon the inner surface of the shell in the interior cavity and is foldable in conformity with at least one fold of the shell.

PRIORITY CLAIMS

This application claims the benefit of U.S. Provisional Application No. 61/385,788, filed Sep. 23, 2010.

TECHNICAL FIELD

The present invention generally relates to of method of preparing a stabilized nitrocellulose article, and more particularly relates to a method of stabilizing nitrocellulose in a nitrocellulose-containing mat of fibers.

BACKGROUND

Long life, stabilized, nitrocellulose (NC) propellants can be found in the form of tough materials which do not lend themselves to mechanical manipulation nor do they have much surface area. The process of forming nitrocellulose propellants includes nitrating a base material to form nitrocellulose, solvating the nitrocellulose, thereby causing the nitrocellulose to form into a gel or liquid (sometimes called a lacquer) and then adding a stabilizer to the nitrocellulose. This material is then extruded, pressed, or forced through a sieve-like screen to particulate the nitrocellulose, which forms it into small beads, sheets, perforated grains, or larger monolithic propellant grains. The nitrocellulose loses mechanical properties of the base material, becomes less flexible, and surface area attributable to the base material is lost once the solvent is removed.

While there are examples of nitrated cloth forms, these examples are not known to be stabilized and typically deteriorate rapidly. Furthermore, the nature of stabilizing nitrocellulose traditionally is accomplished by solvating the nitrocellulose into a gel or liquid (i.e., a lacquer), whereby the nitrated cloth forms lose their original form, mechanical properties, and naturally high surface are.

Accordingly, it is desirable to provide a new method of preparing a stabilized nitrocellulose article having a nitrocellulose content. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

BRIEF SUMMARY

Methods are provided for preparing a stabilized nitrocellulose article. In one embodiment, a method includes providing a nitrocellulose-containing mat of fibers. The nitrocellulose-containing mat of fibers is treated with a solution that comprises a solvent and a stabilizer to produce the stabilized nitrocellulose article. The stabilizer is soluble in the solvent. The solvent dissolves the stabilizer at ambient pressure and temperature. The nitrocellulose in the nitrocellulose-containing mat of fibers is substantially insoluble in the solvent at ambient pressure and temperature.

In another embodiment, a method includes treating a mat of cellulose-containing fibers that is free of nitrocellulose content to a mixture of nitric acid and sulfuric acid to convert cellulose to nitrocellulose, thereby producing a nitrocellulose-containing mat of fibers. The nitrocellulose-containing mat of fibers is treated with a solution that comprises a solvent and a stabilizer to produce the stabilized nitrocellulose article. The stabilizer is soluble in the solvent. The solvent dissolves the stabilizer at ambient pressure and temperature. The nitrocellulose in the nitrocellulose-containing mat of fibers is substantially insoluble in the solvent at ambient pressure and temperature.

An inflatable bag is provided and comprises a shell and the stabilized nitrocellulose article. The shell is formed from a foldable material and has an outer surface and an inner surface. The inner surface defines an interior cavity of the inflatable bag. The stabilized nitrocellulose article is disposed upon the inner surface of the shell in the interior cavity. The stabilized nitrocellulose article is foldable in conformity with at least one fold of the shell. The stabilized nitrocellulose article has a thickness of from 0.012 to 130 mm and comprises a nitrocellulose-containing mat of fibers and a stabilizer content. The stabilized nitrocellulose article is substantially free of damping agents and solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and

FIG. 1 is a cross-sectional schematic side view of a portion of a stabilized nitrocellulose article prepared in accordance with the method described herein;

FIG. 2 is a cross-sectional schematic side view of an inflatable bag including the stabilized nitrocellulose article as a gas generator therefore, the stabilized nitrocellulose article prepared in accordance with the method described herein;

FIG. 2A is a partial cross-sectional schematic side view of a portion of the inflatable bag of FIG. 2;

FIG. 3 is a cross-sectional schematic side view of the bag of FIG. 2 once inflated;

FIG. 3A is a partial cross-sectional schematic side view of a portion of the inflated bag of FIG. 3; and

FIG. 4 is a schematic perspective view of a nitrocellulose-containing mat of fibers being treated with a solution containing a solvent and a stabilizer.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description.

A method of preparing a stabilized nitrocellulose article 10 is provided herein, with the resulting stabilized nitrocellulose article 10 being useful in various applications that may not have previously been possible. The resulting stabilized nitrocellulose article 10 is useful in various applications to provide a source of propellant and/or a gas-generation source. Further, as described in further detail below, due to the flexible nature of the resulting stabilized nitrocellulose articles 10 produced in accordance with the method described herein, the stabilized nitrocellulose articles 10 can be used in applications that involve exposure to stress, bending, or other manipulation of the nitrocellulose article 10 without materially compromising the performance thereof such that the stabilized nitrocellulose articles 10 may offer a viable alternative to existing solid fuel and gas-generation sources.

The instant method includes the step of providing a nitrocellulose-containing mat of fibers. The nitrocellulose-containing mat of fibers may be in woven form, such as a cloth, or may be in non-woven form, such as a card, clump of fibers, or felt. Suitable fibers may be derived from cellulose-containing fibers including cotton, wood fiber, and combinations thereof. However, it is to be appreciated that the fibers may be derived from other cellulose-containing fibers including flax, jute, ramie, and combinations thereof, as well as combinations of such fibers and cotton and/or wood fiber. In one particular embodiment, the fibers in the nitrocellulose-containing mat of fibers are derived from cotton, which has particularly high cellulose content and may enable a desired nitrocellulose content to be achieved in the nitrocellulose-containing mat of fibers while enabling the nitrocellulose-containing mat of fibers to retain physical properties thereof that are akin to properties of the mat of cellulose-containing fibers prior to conversion of cellulose to nitrocellulose. In this embodiment, the mat of cellulose-containing fibers may include, but is not limited to, cotton ball material, cotton cheese cloth, cotton sheet cloth, or cotton paper. Specific examples of mats of cellulose-containing fibers include a coarse weave of cotton fibers (tight weave burlap) natural cotton cloth, a bleached cotton cloth flannel material, a Venus cotton muslin unbleached, and an organic cotton muslin (fine weave) unbleached.

The fibers contained in the mat of cellulose-containing fibers, prior to conversion of cellulose to nitrocellulose, preferably have a relatively high surface area to volume value that is appropriate to maximize the burn rate of the resulting stabilized nitrocellulose article 10 burns that is produced therefrom. The surface area to volume value is dependent upon average thread or fiber diameter. Other factors, such as density and, when the mat is woven, thread count, may also affect the burn rate of the resulting stabilized nitrocellulose article 10. However, relatively high surface area to volume values may also be useful to promote efficient conversion of cellulose to nitrocellulose, which may also contribute to the ability of the resulting stabilized nitrocellulose article 10 to retain physical properties thereof that are akin to properties of the mat of cellulose-containing fibers prior to conversion of cellulose to nitrocellulose, including flexibility properties as described in further detail below. The above properties are typically balanced to achieve a desired burn rate and substantially retain physical properties of the mat of cellulose-containing fibers in the stabilized nitrocellulose article 10. Preferably, surface area to mass of the mat of cellulose-containing fibers is from 0.005 to 0.5 mm, and average fiber diameter is preferably from 0.01 mm to 1.25 mm.

Conversion of cellulose to nitrocellulose is a process that is well known in the art and generally involves treating a mat of cellulose-containing fibers that is free of nitrocellulose content with a nitrating composition of nitric acid and sulfuric acid to convert the cellulose to nitrocellulose. The cellulose may be completely converted to nitrocellulose, or may be partially converted (with some cellulose remaining in the resulting nitrocellulose-containing mat of fibers). In one embodiment, the nitrocellulose-containing mat of fibers may be prepared by and provided from a third party or may be prepared offsite and subsequently shipped for further processing. In this embodiment, the nitrocellulose-containing mat of fibers typically has a damping agent content as a safeguard against inadvertent ignition or combustion of the nitrocellulose-containing mat of fibers. Water is a typical damping agent; however, other damping agents such as alcohols are also known to be suitable under some circumstances (although the type of damping agent may be limited by the effect of such damping agents on dissolving the nitrocellulose). When included, the damping agent content is typically at least 25 weight % (wt. %) based upon the total weight of the nitrocellulose-containing mat of fibers to avoid classification of the nitrocellulose-containing mat of fibers as an explosive, which may be problematic when transporting the nitrocellulose-containing mat of fibers.

In another embodiment, the step of providing the nitrocellulose-containing mat of fibers comprises treating a mat of cellulose-containing fibers that is free of nitrocellulose content with a nitrating composition of nitric acid and sulfuric acid to convert cellulose to nitrocellulose. Although possibly unnecessary (e.g., under circumstances in which the cellulose in the mat of cellulose-containing fibers is converted to nitrocellulose as part of an integral process immediately prior to further processing), the nitrocellulose-containing mat of fibers may be provided with the damping agent in this embodiment as well. In any event, as alluded to above, the nitrocellulose-containing mat of fibers has the nitrocellulose content prior to further processing. When the nitrocellulose-containing mat of fibers has the damping agent, at least a portion of the damping agent is typically removed from the nitrocellulose-containing mat of fibers prior to further processing. For example, damping agent is typically removed to achieve a damping agent content of less than or equal to 5 wt. %, alternatively from 0 to 7.0 wt. %, alternatively from 0.01 to 1.0 wt. % of the damping agent based upon the total weight of the nitrocellulose-containing mat of fibers.

The nitrocellulose content of the nitrocellulose-containing mat of fibers is not particularly limited and it is to be appreciated that the method described herein is effective independent of nitrocellulose content of the nitrocellulose-containing mat of fibers (so long as at least some nitrocellulose is present). However, stabilization may not be required under some circumstances when very low amounts of nitrocellulose are present in the nitrocellulose-containing mat of fibers, and the instant method may be more applicable to circumstances in which sufficient nitrocellulose is present in the nitrocellulose-containing mat of fibers for use in propellant or gas generation applications. The nitrocellulose content of the nitrocellulose-containing mat of fibers is typically measured in mol % of nitrogen based upon the total content of the nitrocellulose-containing mat of fibers, which represents a degree of substitution of cellulose with NO₂. Typically, the nitrocellulose content of the nitrocellulose-containing mat of fibers is from 11.3 to 14 mol % nitrogen, alternatively from 12 to 13.5 mol % nitrogen, alternatively from 11.3-11.8 mol %, alternatively from 11.8 to 12.3 mol %, which enables the nitrocellulose-containing mat of fibers to be useful in propellant and gas generation applications and at which nitrocellulose content the method described herein is particularly useful.

As alluded to above, the nitrocellulose-containing mat of fibers, prior to further treatment, generally has physical properties that are akin to the physical properties of the mat of cellulose-containing fibers prior to conversion of cellulose to nitrocellulose. In one embodiment, as shown in FIG. 1, the nitrocellulose-containing mat of fibers is sufficiently flexible such that the nitrocellulose-containing mat of fibers is foldable upon itself with no visible cracking or material loss at the fold line 12. Thickness of the nitrocellulose-containing mat of fibers may be a factor in whether cracking or material loss occurs upon folding, and appropriate thicknesses of the stabilized nitrocellulose article 10 are described in further detail below, which thicknesses also apply to the nitrocellulose-containing mat of fibers prior to stabilization. Suitable nitrocellulose-containing mat of fibers are commercially available that exhibit no visible cracking or material loss upon folding, even those having nitrocellulose content within the range of from 11.3 to 14 mol % nitrogen; however, it is within the skill of those in the art to produce nitrocellulose-containing mat of fibers within the above-specified range and having the flexibility that is specified above. It is notable that a regimen of rinsing, boiling, and drying the nitrocellulose-containing mat of fibers may be necessary to alleviate stiffness and yellow tinting that may result during the conversion of cellulose to nitrocellulose.

After optional removal of the damping agent from the nitrocellulose-containing mat of fibers, the nitrocellulose-containing mat of fibers is treated with a solution that comprises a solvent and a stabilizer to produce the stabilized nitrocellulose article 10. The stabilizer is soluble in the solvent at ambient temperature and pressure (e.g., about 21° C. and about 1 atm), which enables the stabilizer to be at least partially dissolved to promote introduction of the stabilizer into the nitrocellulose-containing mat of fibers for purposes of stabilizing the nitrocellulose therein. While the stabilizer is soluble in the solvent at ambient temperature and pressure, it is to be appreciated that the stabilizer can be dissolved in the solvent at greater or lesser temperatures and pressures than ambient. The nitrocellulose in the nitrocellulose-containing mat of fibers is substantially insoluble in the solvent at ambient pressure and temperature. With the nitrocellulose in the nitrocellulose-containing mat of fibers being substantially insoluble in the solvent, the physical properties of the nitrocellulose-containing mat of fibers prior to treatment with the solution (at least in terms of flexibility) are not materially changed. More specifically, even after treatment with the solution, the stabilized nitrocellulose article 10 may still be foldable upon itself with no visible cracking or material loss at the fold line 12.

The stabilizer is a sacrificial material that maintains stability of the nitrocellulose by reacting with NO₂ that forms as a result of decomposition of the nitrocellulose over time. Suitable stabilizers for nitrocellulose are known in the art. Examples of suitable stabilizers include amine-based stabilizers, symmetrical or asymmetrical ureas, and combinations thereof. Specific examples of suitable stabilizers include diphenylamine, 2-nitro diphenylamine, ethyl centralite, 1,1-diphenylurea, 1,1-diphenyl-3-methyl-urea, resorcinol, and the like, and combinations thereof. Particularly suitable stabilizers include diphenylamine, 2-nitrodiphenylamine, diphenyl urea, and the like, and combinations thereof.

The stabilized nitrocellulose article 10 typically has a stabilizer content of at least 1.5 mol %, alternatively from 1.5 to 5 wt. %, alternatively from 1.5 to 3 wt. %, alternatively from 2 to 2.5 wt. %, based upon the total weight of the stabilized nitrocellulose article 10. Because some stabilizer may be lost during the treating step, or may be drained off along with solvent, it is to be appreciated that stabilizer load in the solution may be slightly adjusted to achieve a final stabilizer content within the above ranges. Of course, it is to be appreciated that target stabilizer content may be dependent upon nitrocellulose content in the nitrocellulose-containing mat of fibers, and higher stabilizer content in the stabilized nitrocellulose article 10, within the above ranges, may be desirable when the nitrocellulose-containing mat of fibers has higher nitrocellulose content. Further, it is to be appreciated that because the stabilizer is a sacrificial material and is consumed over time as the nitrocellulose decomposes, higher amounts of stabilizer may be desirable to maintain stability of the stabilized nitrocellulose article 10 over time. Typically, the stabilized nitrocellulose article 10 includes sufficient stabilizer content to remain stable for at least 2 years, typically for about 20 years.

Any solvent that satisfies the above-mentioned constraints may be suitable. Preferably, the solvent is an alcohol solvent (i.e., a solvent that is characterized as an alcohol). The alcohol solvent preferably consists of saturated hydrocarbon units and one or more carbon-bonded hydroxyl groups. In one embodiment, the alcohol solvent is further defined as a C₁ to C₆ alcohol solvent, and may be even further defined as a mono-functional alcohol having a C₁ to C₆ hydrocarbon chain. Although primary carbon-bonded hydroxyl groups are preferred, the alcohol solvent is not particularly limited in this regard. Specific examples of suitable C₁ to C₆ alcohol solvents include methanol, ethanol, and combinations thereof. As an alternative to alcohol solvents, suitable solvents that meet the above-referenced solubility (or insolubility) requirements relative to the stabilizer and nitrocellulose may include, but are not limited to, water, acetonitrile, and combinations thereof.

As set forth above, the stabilizer is soluble in the solvent at ambient temperature and pressure. More specifically, the solvent dissolves the stabilizer at ambient pressure and temperature, meaning that at least some stabilizer is dissolvable in solution at ambient temperature and pressure although complete dissolution is preferred. Although not to be interpreted as limiting, the stabilizer typically has a solubility in the solvent of from 0.02 to 1000 g/l, alternatively from 0.5 to 860 g/l, alternatively from 10 to 100 g/l of the solvent at ambient pressure and temperature. Again, as set forth above, although the stabilizer is soluble in the solvent at ambient temperature and pressure, it is to be appreciated that the stabilizer may be actually be dissolved in the solvent at temperatures and pressures other than ambient temperature and pressure. Typically, complete dissolution is achieved, although excess solvent is typically employed beyond what is required to completely dissolve the stabilizer for purposes of providing sufficient amounts of solution to wet the entire nitrocellulose-containing mat of fibers, i.e., to saturate the nitrocellulose-containing mat of fibers with the solution. More specifically, in practice, a desired amount of stabilizer to properly stabilize the nitrocellulose in the nitrocellulose-containing mat of fibers is typically calculated based upon the nitrocellulose content of the nitrocellulose-containing mat of fibers, and the nitrocellulose-containing mat of fibers is treated with solution containing the pre-determined amount of stabilizer. However, the necessary amount of solvent to completely dissolve the pre-determined amount of stabilizer is typically less than the amount of solution that is necessary to completely wet the nitrocellulose-containing mat of fibers. Because complete wetting of the nitrocellulose-containing mat of fibers is desired to achieve uniform stabilization thereof, solvent is typically used in excess in the solution to achieve uniform stabilization of the nitrocellulose-containing mat of fibers.

As set forth above, the nitrocellulose in the nitrocellulose-containing mat of fibers is substantially insoluble in the solvent, and suitable solvents for the instant method are limited in this regard. Solubility of the nitrocellulose in the nitrocellulose-containing mat of fibers in the solvent is typically gauged through simple lab bench tests by immersing a nitrocellulose-containing mat of fibers in a candidate solvent and visually observing the effect of the solvent on the nitrocellulose-containing mat of fibers. If the nitrocellulose-containing mat of fibers develops a gelled or liquid phase therein after submersion in the solvent for 15 minutes at ambient pressures and temperatures, and if the nitrocellulose-containing mat of fibers retains a gelled phase after solvent removal, the nitrocellulose can be considered soluble in the solvent. Although not to be interpreted as limiting, the nitrocellulose typically has a solubility in the solvent of less than 5 g/l, alternatively less than or equal to 0.5 g/l of the solvent at ambient pressure and temperature, which is considered substantially insoluble for purposes of the instant application.

Nitrocellulose content may affect whether the nitrocellulose within a particular nitrocellulose-containing mat of fibers is soluble in the solvent. For example, when the nitrocellulose content is from 11.3 to 11.8 mol % nitrogen, and when the solvent is an alcohol solvent, the nitrocellulose in the nitrocellulose-containing mat of fibers may be more soluble in the alcohol solvent than when the nitrocellulose content in the nitrocellulose-containing mat of fibers is higher. However, even at 11.3 to 11.8 mol % nitrogen, the nitrocellulose in the nitrocellulose-containing mat of fibers still has acceptable insolubility to alcohol solvent, whereas the nitrocellulose may be unacceptably soluble in the alcohol solvent at nitrocellulose contents below 11.3 mol % nitrogen.

Typically, the solution is free of additional solvents within which the nitrocellulose in the nitrocellulose-containing mat of fibers is soluble. However, it is to be appreciated that the solution may comprise additional components so long as the nitrocellulose in the nitrocellulose-containing mat of fibers remains insoluble in the solution.

Preferably, the step of treating the nitrocellulose-containing mat of fibers with the solution, as described above, is the step that first introduces the stabilizer into the nitrocellulose-containing mat of fibers, with the nitrocellulose-containing mat of fibers being free of stabilizer prior to such treatment. In this regard, at least a portion of the stabilizer is preferably dissolved in the solvent prior to treating the nitrocellulose-containing mat of fibers with the solution, stabilizer is preferably absent from the nitrocellulose-containing mat of fibers prior to treatment with the solution, and at least some of the stabilizer is preferably introduced into the nitrocellulose-containing mat of fibers in dissolved form.

The particular manner in which the nitrocellulose-containing mat of fibers is treated with the solution is not particularly limited, although process efficiency may be a consideration. In particular, the nitrocellulose-containing mat of fibers can be treated with the solution by immersing the nitrocellulose-containing mat of fibers in a vessel that contains the solution. However, to enhance processing efficiency and enable continuous processing, as shown in FIG. 4, the step of treating the nitrocellulose-containing mat of fibers with the solution may be further defined as passing the nitrocellulose-containing mat of fibers over a perforated surface 26 and introducing the solution into the nitrocellulose-containing mat of fibers through the perforations 28 in the perforated surface 26. As also shown in FIG. 4, the nitrocellulose-containing mat of fibers may be provided in a roll 30, and the perforated surface 26 may be a roller 26 that has perforations 28 through which the solution is pumped. However, it is to be appreciated that the perforated surface 26 may be provided in a form other than a roller 26 so long as the nitrocellulose-containing mat of fibers is passable over the perforated surface 26 to enable the solution to be introduced into the mat.

After the stabilized nitrocellulose article 10 is produced, i.e., after the nitrocellulose-containing mat of fibers is treated with the solution, the solvent is substantially removed from the stabilized nitrocellulose article 10. In one embodiment, the nitrocellulose article is exposed to ambient temperature and pressure for a time sufficient to allow the solvent to vaporize. In another embodiment, the solvent is removed by drying the stabilized nitrocellulose article 10 under vacuum and/or temperatures that are higher than ambient temperature.

The stabilized nitrocellulose article 10 prepared in accordance with the method described herein typically has a thickness of from 0.012 to 130 mm, alternatively from 0.012 to 30 mm, which is sufficiently thick to enable the stabilized nitrocellulose article 10 to be employed in propellant and gas generation applications, but is still sufficiently thin to enable manipulation without experiencing cracking or material loss. More specifically, the stabilized nitrocellulose article 10 is foldable upon itself (e.g., folded at a 180° angle) with no visible cracking or material loss at the fold line 12. Stabilized nitrocellulose articles 10 that are foldable without exhibiting visible cracking or material loss, as determined through the above tests, are useful in various applications that require propellant or gas generating articles 10 that can be manipulated. Particularly suitable applications for the stabilized nitrocellulose articles 10 described herein include inflatable bags.

Referring to FIGS. 2, 2A, 3, and 3A, inflatable bags 14 that can benefit from use of the stabilized nitrocellulose article 10 that is formed in accordance with the method described herein include a shell 16 and the stabilized nitrocellulose article 10. As shown in FIG. 2, the shell 16 is formed from a foldable material to enable the inflatable bag 14 to be compressed, which is advantageous for both storage and utility of the inflatable bag 14. The foldable material generally provides the structural properties of the inflatable bag 14. The foldable material need not necessarily be a flexible material; suitable foldable materials include metals, polymers, fibers, and combinations thereof, and the shell 16 may be a sheet of the foldable material, a fabric of the foldable material, etc. In various embodiments, the foldable material can be metals such as steel, aluminum, tin, copper, etc. Alternatively, the foldable material can be a polymer such as Kevlar®, Vectra®, or similar materials that exhibit high strength. In this regard, the inflatable bag 14 in which the shell 16 is formed from such foldable materials can be used for a range of applications including, but not limited to, structural functions and impact-absorption functions.

The shell 16 has an outer surface 18 and an inner surface 20 that defines an interior cavity 22 of the inflatable bag 14. Because the bag 14 is inflatable, the interior cavity 22 is typically sealed from the ambient environment. However, it is to be appreciated that the interior cavity 22 may be partially open to the ambient environment so long as sufficient pressure can be built within the interior cavity 22 to inflate the bag 14.

While the inner surface 20 of the shell 16 defines the interior cavity 22, it is to be appreciated that additional layers may be present within the interior cavity 22 and disposed upon the inner surface 20 of the shell 16. For example, an optional silicone layer 24 may be disposed on the inner surface 20 of the shell 16 in the interior cavity 22 to partially shield the shell 16 from heat of combustion during gas generation.

As set forth above, the stabilized nitrocellulose article 10 is disposed upon the inner surface 20 of the shell 16 in the interior cavity 22. In this regard, the stabilized nitrocellulose article 10 may be disposed directly upon the inner surface 20 of the shell 16. Alternatively, when one or more additional layers, such as the silicone layer 24, are disposed on the inner surface 20 of the shell 16, the one or more additional layers may be disposed between the inner surface 20 of the shell 16 and the stabilized nitrocellulose article 10. The stabilized nitrocellulose article 10 that is employed in the inflatable bag 14 is typically substantially free of damping agents and solvent (which may be removed during preparation of the stabilized nitrocellulose article 10 in accordance with the method that is described above).

As best shown in FIG. 2A, the stabilized nitrocellulose article 10 is foldable in conformity with at least one fold of the shell 16, under which circumstances the beneficial properties of the particular stabilized nitrocellulose articles 10 described herein can be exploited. Typically, the stabilized nitrocellulose article 10 is in conformity with the entire inner surface 20 of the shell 16 to spread heat of combustion over the inner surface 20 of the shell 16 and minimize the possibility of a burn through of the shell 16.

To inflate the inflatable bag 14, one end of the stabilized nitrocellulose article 10 in the interior cavity 22 may be ignited, with the flame front propagating through the length of the interior cavity 22 at a rate governed by the properties of the stabilized nitrocellulose article 10. Ignition of the stabilized nitrocellulose article 10 generates gas at high pressures to expand the shell 16 and form an inflated bag 14 as shown in FIG. 3. As shown in FIG. 3A, after ignition of the stabilized nitrocellulose article 10, the stabilized nitrocellulose article 10 is typically consumed (as evidenced by the absence of the stabilized nitrocellulose article 10), with the stabilized nitrocellulose article 10 absent from the resulting inflated bag 14. Of course, it is to be appreciated that residues from the ignition of the stabilized nitrocellulose article 10 may remain within the inflated bag 14. Because the stabilized nitrocellulose article 10 generates the gas that is necessary to inflate the bag 14, the inflatable bag 14 may be free of a gas generator housing when the stabilized nitrocellulose article 10 is formed from a mat of cellulose-containing fibers having an appropriate surface area to mass value as described above. Gas generator housings typically add expense and bulk to known inflatable bags, but are typically beneficial when burn-through of the shell 16 is a risk. However, because the stabilized nitrocellulose articles 10 can be folded within the inflatable bag 14 and spread heat of combustion over the inner surface 20 of the shell 16, risk of burn-through can be minimized. This is an advantage over propellants that must be burned in gas generator housings and that necessarily insert the hot inflation gas at a single point in the bag, resulting in the potential for a burn through. The stabilized nitrocellulose article 10 also overcomes challenges of systems that require a gas generator housing in that gas generator housings typically add significant weight to the inflatable bag, and such added weight is eliminated through the use of the stabilized nitrocellulose article 10 as described herein.

The following Examples are intended to illustrate the method of preparing the stabilized nitrocellulose article 10 as described herein, and are not to be viewed as limiting to the invention.

EXAMPLES

Mats of cellulose-containing fibers were first provided and cellulose in the mats was converted to nitrocellulose. More specifically, the following mats of cellulose-containing fibers were obtained:

1. Cotton cheesecloth, multi-layer (white); 17″×30″, 11.5 g

2. Cotton balls (white); 11.7 g

3. Fabric, tight weave burlap; 8″×11″, 11.5 g (Osnaburg Natural)

4. Fabric, tight weave off white 9″×13″, 11.1 g (Born from Natural Organic)

A nitrating composition of 68.9% sulfuric acid, 16.05% fuming nitric acid (90%), 15.05% standard nitric acid (70.5%) was prepared as follows:

Composition Sulfuric Acid (g) Fuming Nitric (g) Standard Nitric (g) 1 690.4 161.3 152.5 2 689.1 161.0 151.0

The mats of cellulose-containing fibers were added to the nitrating compositions and were soaked under stirring for about 30 minutes to form nitrocellulose-containing mats of fibers. The nitrocellulose-containing mats of fibers became somewhat rigid during nitration. The nitrocellulose-containing mats of fibers were rinsed 4 times with distilled water and were boiled for 4 hours in distilled water with about 1 g of sodium bicarbonate added. The nitrocellulose-containing mats of fibers having the nitrocellulose content were dried by spreading them out on a mesh pan, placing the pan in a fume hood and drying at ambient temperature (overnight).

All nitrocellulose-containing mats of fibers had a slightly yellow tint. All of the nitrocellulose-containing mats of fibers were flexible, but cheesecloth was the most flexible. The cotton balls were unwound into strips. The fabrics stiffened; the finest cloth (Born From Nature) had tears.

Four solvent candidates were selected for dissolving diphenylamine (DPA, ACS from Alpha Aesar (large crystals; flakes)) stabilizer: n-butyl acetate, ethyl lactate, methanol, and ethyl alcohol. Before trying to dissolve the DPA, the solvent candidates were applied to nitrocellulose-containing mats of fibers, prepared as described above, to observe the effects. The n-butyl acetate and ethyl lactate both dissolved the nitrocellulose in the nitrocellulose-containing mats of fibers and turned it to gel. After removing the n-butyl acetate and ethyl lactate by vacuum, the nitrocellulose-containing mats of fibers were rubbery/gooey. The methanol wetted the nitrocellulose in the nitrocellulose-containing mats of fibers without dissolving it, but the nitrocellulose-containing mats of fibers were found to be slightly sticky after drying. The ethyl alcohol wetted the nitrocellulose in the nitrocellulose-containing mats of fibers without dissolving it, and returned the sample to its original state (flexible and dry) after drying.

Next the DPA was dissolved in the methanol and ethyl alcohol. The DPA was first ground and screened with a 200 mesh sieve before being added to the solvents. The methanol appeared to wet the crystal of the DPA (the literature indicates DPA has a solubility of 474 g/l of methanol). The ethyl alcohol appeared to dissolve the DPA. Thereafter, the DPA was taken straight from its bottle (not ground) to be dissolved in the ethyl alcohol. The amount of ethyl alcohol was adjusted to just saturate the nitrocellulose-containing mats of fibers. The amount of DPA was determined by weight to achieve a 2 weight % (wt. %) ratio with the nitrocellulose remaining after the ethyl alcohol was evaporated. More specifically, the nitrocellulose-containing mats of fibers were stabilized as follows:

Addition of DPA to Cheesecloth: 1.8047 g sample of cheesecloth having nitrocellulose content and prepared as described above (5″×3.5″ double layer); 0.0362 g DPA (˜2%) used straight from the bottle. The DPA was dissolved in 7.264 g (˜10 ml) ethyl alcohol. The cheesecloth was added to the DPA/ethyl alcohol solution and stirred until all of the solution was absorbed into the cheesecloth. An additional 2.043 g ethyl alcohol was added, and the cheesecloth became saturated, leaving about 1 ml un-absorbed. A stabilized nitrocellulose article was thus formed. The stabilized nitrocellulose article was vacuum dried at ambient temperature.

Addition of DPA to cotton balls: 1.8596 g sample of unwound cotton balls having nitrocellulose content and prepared as described above (re-weighed: 1.8605 g); 0.0372 g DPA used straight from the bottle. The DPA was dissolved in 9.455 g of ethyl alcohol and 1.86 g of cotton balls having nitrocellulose content were added thereto. The DPA/ethyl alcohol solution was totally absorbed with some dry spots remaining in the cotton balls. An additional 4.4 grams of ethyl alcohol was added to saturate the cotton balls. A stabilized nitrocellulose article was thus formed. The stabilized nitrocellulose article was vacuum dried at ambient temperature. The stabilized nitrocellulose articles were slightly stiff, but easily folded and manipulated as prior to stabilization, with no visible cracking or loss of material at the fold line.

Initial testing confirmed that the samples had been stabilized, and the samples were placed into an oven set at 140° F. (60° C.) for accelerated long term aging. Samples of un-stabilized cotton ball having a nitrocellulose content, un-stabilized cheese cloth having a nitrocellulose content, and stabilized samples of each were prepared in vials. Initial weight was recorded for each sample. The un-stabilized samples began to show reaction at about 24 hours. The stabilized samples began to show early signs of reaction at 1,010 hours. By the Arrhenius Equation this amount of time is equivalent to 1.8 years at 68° F. (20° C.).

A Bomb Calorimetry test was performed on a sample of stabilized cheese cloth having the nitrocellulose content. The result was 960.98 Cal/gram indicating a proper nitration of between 12.5 and 13 mol % nitrogen.

Another sample of stabilized cotton cheesecloth having the nitrocellulose content was independently tested for nitrocellulose content and DPA content using a titration method that is routinely used to determine the percent nitration of ball powder. In particular, 15 g of cotton cheesecloth having nitrocellulose content and prepared as described above was provided. DAP was weighed and placed into a beaker, with ethyl alcohol subsequently added. The DPA was dissolved in the ethyl alcohol. The cheesecloth was placed in a Teflon pan and the DPA/ethyl alcohol solution was poured over the cheesecloth in 20 ml aliquots. The cheesecloth was turned and kneaded to ensure saturation thereof. Some excess solution remained in the pan. A stabilized nitrocellulose article was thus formed. The stabilized nitrocellulose article was spread onto a pan and placed in a fume hood to dry, with a fan running and air only moving across the surface of the stabilized nitrocellulose article. The final weight of the stabilized nitrocellulose article was 15.3175 g; the theoretical total weight (Cheesecloth+DPA)=15.048 g+0.307 g=15.355 g; Calculated DPA (assumes initial cheesecloth weight was correct)=15.3175 g−15.048 g=0.2695 g (1.75%). The independent tests of the cheesecloth confirmed a 12.88 mol % nitration. A HPLC instrument was also used to determine the percent of DPA and found it to be 1.62% DPA in one sample tested. The 1.62% of DPA, being less than the targeted 2% is not significant. To shorten the gap between the targets DPA content and the actual DPA content, additional rinsing/boiling (pouching) procedures may possibly be employed to ensure removal of acids and salts from the nitrocellulose-containing mat of fibers.

Samples of each of the nitrated material were burned to determine what effect the form and weave might have on the burn rate. Each burned rapidly and completely. Relative speeds (fastest to slowest): Cotton balls, Cheesecloth, course weave natural fabric, fine weave organic muslin.

While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention. It being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims. 

1. A method of preparing a stabilized nitrocellulose article, said method comprising the steps of: providing a nitrocellulose-containing mat of fibers; treating the nitrocellulose-containing mat of fibers with a solution comprising solvent and a stabilizer that is soluble in the solvent to produce the stabilized nitrocellulose article; wherein the solvent dissolves the stabilizer at ambient pressure and temperature and wherein the nitrocellulose in the nitrocellulose-containing mat of fibers is substantially insoluble in the solvent at ambient pressure and temperature.
 2. The method of claim 1 wherein at least a portion of the stabilizer is dissolved in the solvent prior to treating the nitrocellulose-containing mat of fibers with the solution.
 3. The method of claim 1 wherein the solution is free of additional solvents within which the nitrocellulose in the nitrocellulose-containing mat of fibers is soluble.
 4. The method of claim 1 wherein the nitrocellulose content of the nitrocellulose-containing mat of fibers, as measured in mol % nitrogen based upon the total content of the nitrocellulose-containing mat of fibers, is from 11.3 to 14 mol % nitrogen.
 5. The method of claim 1 wherein the nitrocellulose-containing mat of fibers has a damping agent content and wherein the damping agent content is at least 25 weight % based upon the total weight of the nitrocellulose-containing mat of fibers.
 6. The method of claim 5 wherein at least a portion of the damping agent is removed from the nitrocellulose-containing mat of fibers prior to treating the nitrocellulose-containing mat of fibers with the solution.
 7. The method of claim 1 wherein the step of providing the nitrocellulose-containing mat of fibers comprises treating a mat of cellulose-containing fibers free of nitrocellulose content with a mixture of nitric acid and sulfuric acid to convert cellulose to nitrocellulose.
 8. The method of claim 1 wherein the stabilized nitrocellulose article has a stabilizer content of at least 1.5 mol %.
 9. The method of claim 1 wherein the solvent is further defined as a C₁ to C₆ alcohol solvent.
 10. The method of claim 9 wherein the C₁ to C₆ alcohol solvent comprises an alcohol selected from the group consisting of methanol, ethanol, and combinations thereof.
 11. The method of claim 1 wherein the stabilizer comprises a material selected from the group consisting of diphenylamine, 2-nitrodiphenylamine, diphenyl urea, and combinations thereof.
 12. The method of claim 1 wherein the step of treating the nitrocellulose-containing mat of fibers with the solution is further defined as passing the nitrocellulose-containing mat of fibers over a perforated surface and introducing the solution into the nitrocellulose-containing mat of fibers through the perforations in the perforated surface.
 13. The method of claim 1 wherein the solvent is substantially removed from the stabilized nitrocellulose article.
 14. A stabilized nitrocellulose article prepared in accordance with the method of claim
 13. 15. The stabilized nitrocellulose article of claim 14 having a fiber thickness of from 0.012 to 130 mm.
 16. A method of preparing a stabilized nitrocellulose article, said method comprising the steps of: treating a mat of cellulose-containing fibers free of nitrocellulose content to a mixture of nitric acid and sulfuric acid to convert cellulose to nitrocellulose, thereby producing a nitrocellulose-containing mat of fibers; treating the nitrocellulose-containing mat of fibers with a solution comprising a solvent and a stabilizer that is soluble in the solvent to produce the stabilized nitrocellulose article; wherein the solvent dissolves the stabilizer at ambient pressure and temperature and wherein the nitrocellulose in the nitrocellulose-containing mat of fibers is substantially insoluble in the solvent at ambient pressure and temperature.
 17. The method of claim 16 wherein at least a portion of the stabilizer is dissolved in the solvent prior to treating the nitrocellulose-containing mat of fibers with the solution.
 18. The method as set forth in claim 16 wherein the solution is free of additional solvents within which the nitrocellulose in the nitrocellulose-containing mat of fibers is soluble.
 19. The method as set forth in claim 16 wherein the nitrocellulose content of the nitrocellulose-containing mat of fibers, as measured in mol % nitrogen based upon the total content of the nitrocellulose-containing mat of fibers, is from 11.3 to 14 mol % nitrogen.
 20. An inflatable bag comprising: a shell formed from a foldable material, said shell having an outer surface and an inner surface defining an interior cavity of said inflatable bag; and a stabilized nitrocellulose article disposed upon said inner surface of said shell in the interior cavity and foldable in conformity with at least one fold of said shell, said stabilized nitrocellulose article having a thickness of from 0.012 to 130 mm and comprising a nitrocellulose-containing mat of fibers and a stabilizer content, wherein said stabilized nitrocellulose article is substantially free of damping agents and solvent. 